Note 4 - Stockholders' Equity - Options Outstanding Separated into Range of Exercise Prices (Details) - $ / shares	9 Months Ended
	Sep. 30, 2016	Dec. 31, 2015
Range One [Member]
Lower range (in dollars per share)	$ 0.901
Upper range (in dollars per share)	$ 1.576
Options outstanding (in shares)	805,988
Weighted average remaining contractual life	2 years 188 days
Weighted average exercise price (in dollars per share)	$ 1.281
Options exercisable (in shares)	805,988
Weighted average remaining contractual life	2 years 188 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 1.281
Range Two [Member]
Lower range (in dollars per share)	6.23
Upper range (in dollars per share)	$ 8.80
Options outstanding (in shares)	90,873
Weighted average remaining contractual life	5 years 81 days
Weighted average exercise price (in dollars per share)	$ 6.951
Options exercisable (in shares)	90,873
Weighted average remaining contractual life	5 years 81 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 6.951
Range Three [Member]
Lower range (in dollars per share)	12.47
Upper range (in dollars per share)	$ 16.87
Options outstanding (in shares)	459,166
Weighted average remaining contractual life	7 years 109 days
Weighted average exercise price (in dollars per share)	$ 13.764
Options exercisable (in shares)	275,764
Weighted average remaining contractual life	6 years 151 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 12.47
Range Four [Member]
Lower range (in dollars per share)	19.51
Upper range (in dollars per share)	$ 21.66
Options outstanding (in shares)	554,457
Weighted average remaining contractual life	7 years 318 days
Weighted average exercise price (in dollars per share)	$ 21.207
Options exercisable (in shares)	278,018
Weighted average remaining contractual life	7 years 293 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 21.204
Range Five [Member]
Lower range (in dollars per share)	22.01
Upper range (in dollars per share)	$ 24.23
Options outstanding (in shares)	297,128
Weighted average remaining contractual life	7 years 201 days
Weighted average exercise price (in dollars per share)	$ 22.277
Options exercisable (in shares)	173,799
Weighted average remaining contractual life	7 years 197 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 22.317
Range Six [Member]
Options outstanding (in shares)	154,200
Weighted average remaining contractual life	8 years 144 days
Weighted average exercise price (in dollars per share)	$ 29.88
Options exercisable (in shares)	61,045
Weighted average remaining contractual life	8 years 144 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 29.88
Range Seven [Member]
Options outstanding (in shares)	154,922
Weighted average remaining contractual life	7 years 174 days
Weighted average exercise price (in dollars per share)	$ 39.86
Options exercisable (in shares)	98,920
Weighted average remaining contractual life	7 years 174 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 39.86
Options outstanding (in shares)	2,516,734
Weighted average remaining contractual life	5 years 338 days
Weighted average exercise price (in dollars per share)	$ 14.759	$ 14.026
Options exercisable (in shares)	1,784,407
Weighted average remaining contractual life	5 years 16 days
Weighted average exercise price of options exercisable (in dollars per share)	$ 11.569